There is growing interest in replacing current petroleum based fuels with cleaner renewables such as fatty acid alkyl esters (FAAEs) otherwise known as biodiesel. Biodiesel's biodegradability, low-toxicity, high lubricity, and lower emission profile compared to similar fuels make it one of leading contenders to replace petroleum based products.
Biodiesel is typically produced by catalytically reacting vegetable and/or animal fats with short-chain alcohols to produce biodiesel through transesterification or alcoholysis. Traditional transesterification produces a mix of mono-alkyl esters of long chain fatty acids and several by-products including unpurified glycerol. A lipid transesterification production process is then used to convert the base oil to the desired esters.
Free fatty acids (FFAs) in the base oil are either converted to soap and removed from the process, or they are esterified downstream (yielding more biodiesel) using an acidic catalyst. After processing, biodiesel has combustion properties very similar to those of petroleum diesel, and can replace it in most current uses.
One potentially valuable by-product of the transesterification process is the production of glycerol (also commonly referred to as glycerin). In fact, every 1 ton of biodiesel results in the production of about 100 kg of crude glycerol (typically containing ˜20% water and catalyst residues). Unfortunately, it is very expensive to purify this crude glycerol into a purified form that can be used directly or as a building block for chemical or pharmaceutical uses.
Although optimized over the years, the basic process and chemistry of biodiesel production remains relatively unchanged. For instance U.S. Pat. No. 4,164,506 discloses a traditional method of biodiesel synthesis employing acid catalysis of fatty acids, while the conversion of triglycerides with base catalysts is described in U.S. Pat. Nos. 2,383,601 and 2,494,366. One of the major deficiencies with prior art chemical production methods is their reliance on the use expensive feedstocks containing low levels of FFA (typically below about 2 weight percent, often less than about 0.5 weight percent) and having lower water content.
Current art systems also employ homogenous strong base or acid catalysts that are corrosive, produce large amounts of waste water and require extensive and costly pretreatment and/or downstream processing.
Although some more recent methods convert both free fatty acids and triglycerides to biodiesel they do not teach simultaneous chemical esterification and transesterification in the same reaction chamber. Instead such systems teach carrying out esterification and transesterification steps in separate reaction chambers as part of multi-step processes. The systems often employ multi-stage approaches to avoid the various problems associated feedstocks containing significant amounts of FFA. Other such systems employ bio-based enzymes in place of chemical reaction systems. See, U.S. Pat. Nos. 5,697,986, and 5,713,965 which are hereby incorporated by reference.
As shown in FIG. 4, traditional chemical processes that employ strong bases such as sodium or potassium hydroxide react with FFA to produce undesirable by-products like soap which lead to decreased biodiesel yields and significant increases in downstream purification costs.
Likewise typical acid catalysts are not suitable for processing feedstock containing FFA for multiple reasons including: (1) an excessive amount of acid catalyst is typically required to convert such feedstocks to biodiesel and (2) since the acids need to be neutralized before processing of biodiesel, the increased level of catalyst results in excessive salt generation which fouls the glycerol.
Therefore, a new method for producing biodiesel which can efficiently process a wide variety of feedstocks, including low cost feedstocks containing high levels of FFA is needed.